Slightly off topic: I have the impression that many kids quit with aikido, when they become teens. I guess that when they enter puberty, which is quite a change in their lives, they want to change their pastime activities, too.
Do you others here have the same experience?

It happens in all martial arts schools. Children change as they grow up. During my karate years, I often saw kids start like tykes, doing awesome both in kata and kumite. And then, as soon they reached their early teenage years, they changed, became lazy and and unmotivated. Then they would quit. It happened so often - although not systematically - that I was against starting them at a very young age and I advised parents to wait until they were a bit older.
Another problem was precisely when they had to graduate from the kid's class to the adult class. Some of them struggled for a while before adapting. Others found the new standards offered by older and more mature partners overwhelming, even for kata. And it was not a belt problem. The belt system was the same for the whole school.
My own nephew started taekwondo - in the United States - at age eight and made it to black belt two years later. Then he quit.
How do more experienced instructors tackle these problems?